1. Field of the Invention
The present invention relates to a failure detection apparatus for an alternator (three-phase alternator), for example, mounted on a vehicle. In particular, it relates to a technique for diagnosing a short circuit failure of a rectifier diode group (hereinafter also referred to simply as “diodes”) in a full-wave rectifier circuit in an alternator.
2. Description of the Related Art
A known failure detection apparatus for an alternator detects a voltage duty of a phase voltage detection terminal (hereinafter referred to as a “P terminal”) during the generation of electricity, and when the voltage duty thus detected is in the vicinity of 50%, it is determined that the alternator is normal, whereas when the voltage duty deviates from 50%, it is determined that the alternator is abnormal (see, for example, a first patent document: Japanese patent application laid-open No. H8-65914).
In such a known failure detection apparatus for an alternator, it is necessary to provide voltage dividing resistors of high precision as a voltage detection means for the purpose of accurately detecting whether the voltage duty is in the vicinity of 50%, so there has been a problem that circuitry including such high precision resistors becomes expensive.
In addition, the detection accuracy of the known failure detection apparatus at low electrical load is poor, and hence it is also necessary to provide a cancellation circuit for inhibiting voltage detection at low electrical load, so there has also been a problem that the circuitry becomes more complicated and expensive.